A known vacuum pumping arrangement comprises a turbomolecular pumping mechanism connected in series with a molecular drag pumping mechanism, the latter of which can be of any suitable type such as a Holweck or a Gaede type pumping mechanism. A backing pump is generally provided to reduce pressure at the exhaust of the arrangement and may be of any suitable type such as a regenerative pump or claw pump.
The turbomolecular pumping mechanism comprises one or more circumferential arrays of angled blades supported at a generally cylindrical rotor body. During normal operation, the rotor, which is coupled to a drive, shaft, is rotated between 20,000 and 200,000 revolutions per minute, during which time the rotor blades collide with molecules in a gas urging them towards the pump outlet. The molecular drag pumping mechanism comprises a rotor, which may comprise a hollow cylinder in a Holweck type pumping mechanism, coupled to the drive shaft for simultaneous rotation with the turbomolecular pumping mechanism. Rotation of the molecular drag pumping mechanism imparts a velocity to gas molecules entering it from the exhaust of the turbomolecular pumping mechanism tangentially to the circumference of the cylinder and along spiral channels formed between a stator and the cylinder towards the drag outlet.
The cylinder of a Holweck type pumping mechanism extends generally axially with a circumference about the axis of the drive shaft, and is supported by an apertured plate extending radially from the drive shaft between the turbomolecular pumping mechanism and the cylinder. Therefore, in use, gas passes from the outlet of the turbomolecular pumping mechanism, through the apertured plate and into the molecular drag pumping mechanism.
It is desirable to provide an improved vacuum pumping arrangement.